1. Field
The disclosed embodiments relate generally to IP telephony.
2. Background
Mobile communication devices such as cellular telephones may have more than one air interface. In one example, a cellular telephone is able to communicate conventionally over relatively long distances with a cellular telephone network using a CDMA (Code Division Multiple Access) transceiver. The CDMA transceiver of the cellular telephone communicates with a cellular BTS (Base Transmitter Site) on the cellular telephone network. In addition, the cellular telephone is able to communicate over relatively short distances with a wireless local area network (LAN) using an IEEE 802.11 transceiver. The 802.11 transceiver of the cellular telephone communicates wirelessly with an access point on the LAN.
A first party may use the cellular telephone to place call to a second party using VoIP (voice over Internet Protocol) technology. Voice data is communicated in IP packets from the cellular telephone, over the 802.11 wireless link to the access point, through the LAN, and across the Internet to the second party. When the first party is engaged in such a call, the first party may wish to move away from the 802.11 access point so that the 802.11 communication link is broken. In such a situation, it is desired that the call not be dropped but rather that the call be automatically switched to use longer range cellular telephone network so that the call can be continued using the cellular telephone network.
U.S. Patent Application Publication 2004/0264410 to Sagi et al. discloses using SIP (Session Initialization Protocol) to set up a first VoIP (Voice over Internet Protocol) call between a first communication device and a second communication device. A SIP INVITE message passes from the first communication device, across a wireless link between the first communication device and a wireless access point on a wireless LAN, through an enterprise server, and to a second communication device. When the first communication device begins to move outside the coverage area of the wireless LAN, the enterprise server places a new call to a private number associated with the first communication device on a cellular WAN (Wide Area Network). The new call involves a conventional circuit-switched link between the first communication device and a BTS (cellular Base Transmitter Site) of the cellular WAN. Once the new call is set up using a convention call setup procedure, the enterprise server establishes a three-way conference call involving the first call and the new call. The enterprise server then terminates the link over the wireless LAN. The result is a call that includes a circuit-switched link from the first communication device to the cellular BTS. The resulting call is undesirable in some respects because using the circuit-switched link involves reserving a dedicated amount of bandwidth even if the call that is the subject of the handoff requires a fluctuating amount of bandwidth or only requires a small amount of bandwidth. Where the reservation of an excess amount of unused bandwidth involves added cost, the cost of the handed off call is made undesirably high.
Not only can the cost of the handed off call be undesirably high, but the special enterprise server is required. Providing and maintaining such an enterprise server can be costly. Moreover, the handoff method cannot be practiced in regimes where no such enterprise server has been deployed. Even if an enterprise server is provided, it is required that the two calls both pass through the enterprise server. It is possible that the user of the first communication device may roam into a coverage area of a cellular network where the second call would not pass through the enterprise server. The call handoff method cannot therefore be practiced. A solution is desired.